The invention relates to a 2-laser optical head having at least a semiconductor laser device for recording, a semiconductor laser device for reproduction, a beam splitter for separating and combining two light beams emitted from the semiconductor laser devices, and one objective lens and, more particularly, to a recording/reproducing apparatus using a construction which satisfies a light-using ratio and polarizing characteristics which is necessary for an optical head and is used for separating and combining a light beam for recording and a light beam for reproduction and a 2-laser optical head using the same.
FIG. 2 is a constructional diagram showing a conventional 2-laser optical head. The 2-laser optical head in FIG. 2 is constructed in a manner such that an optical system 300 using a semiconductor laser device 6 of a long oscillation wavelength .lambda..sub.2 (for example, .lambda..sub.2 =780 nm) is used for rewriting an information signal, an optical system 301 using a semiconductor laser device 1 of a short oscillation wavelength .lambda..sub.1 (for example, .lambda..sub.1 =680 nm) is used for reproducing an information signal, and a spot 25 for rewriting is located on the same track 5b on a magneto-optic disk 5 as an information recording medium so as to be preceding to a spot 26 for reproduction by only a predetermined interval. In the 2-laser optical head mentioned above, a dichroism beam splitter 3 for separating and combining laser beams emitted from the optical system 300 for rewriting and the optical system 301 for reproduction needs to keep a light using ratio (the light of the wavelength .lambda..sub.1 is transmitted and the light of the wavelength .lambda..sub.2 is reflected) and to satisfy polarizing characteristics. In an optical head (reproducing optical system 301 in the 2-laser optical head) for reproducing information from a magneto-optic disk 5, a phase difference (phase difference between the P polarization light and the S polarization light) which deteriorates a polarizing state becomes a cause to deteriorate the quality of information signal. It is, therefore, necessary to suppress the phase difference of the optical head to a predetermined value or less. The value of such a phase difference cannot unconditionally be specified due to a performance which is required from the optical disk system to the optical head. However, it is generally required to set the phase difference of the optical head to a value less than 30.degree. to 10.degree.. When it is now assumed, accordingly, that the phase difference which is severely required for the reproducing optical system 301 of the 2-laser optical head is set to 10 degrees or less, it is necessary that a phase difference .delta..sub.1 of the dichroism beam splitter 3 as one of the optical parts which construct the 2-laser optical head is set to be 5.degree. or less.
FIG. 3 is a diagram showing the phase difference .delta..sub.1 of the dichroism beam splitter 3 and the angle dependency of an incident light beam. An axis of abscissa denotes the wavelength .lambda..sub.1 and an axis of ordinate indicates the phase difference .delta..sub.1. As will be understood from the diagram, the incident angle of 45.degree. which is generally used doesn't satisfy the specification (.delta..sub.1 .ltoreq..vertline.5.degree..vertline.) of the phase difference in the wavelength which is used (within a range of .+-.20 nm for the center wavelength of 680 nm). Therefore, in case of using the dichroism beam splitter, it is necessary to set the incident angle to 10.degree. which satisfies the specification of the phase difference.
A construction of the 2-laser optical head of the conventional example will now be described in detail hereinbelow.
A light beam 100 emitted from the semiconductor laser device 1 (wavelength .lambda..sub.1) is transmitted through a beam splitter 2 and passes through the dichroism beam splitter 3 having wavelength selectivity. An incident angle (relative angle between a normal vector 3a and the incident light beam 101) of the incident light beam 101 to the dichroism beam splitter 3 is equal to 10.degree. smaller than 45.degree.. A light beam 102 which was transmitted through the dichroism beam splitter 3 is reflected by a mirror 15 and, after that, is irradiated as a spot 26 for reproduction onto the track 5b of the magneto-optic disk 5 as an (magneto-optical information recording medium by an objective lens 4. A reflection light beam 103 from the disk 5 passes through the objective lens 4 and the mirror 15 and is transmitted through the dichroism beam splitter 3 and is reflected by the beam splitter 2. After that, the reflected light enters a detection optical system 50. On the other hand, a light beam 104 emitted from the semiconductor laser device 6 (wavelength .lambda..sub.2) is transmitted through a beam splitter 7. A transmission light beam 105 enters the dichroism beam splitter 3 at the same incident angle 10.degree. as that of the light beam 101. Therefore, after the light beam 105 was reflected by the dichroism beam splitter 3 in the same direction as that of the light beam 102, it is irradiated onto the track 5b of the disk 5 as a spot 25 for recording (rewriting) by the objective lens 4. A reflection light beam 106 passes through the objective lens 4 and the mirror 15 and is again reflected by the dichroism beam splitter 3 and is, further, reflected by the beam splitter 7. After that, the reflected light beam enters a detection optical system 51.
As another prior art, there is "Three-beam Overwritable magneto-optic disk drive" published in "Optical Data Storage", SPIE (The International Society for Optical Engineering), Vol. 1499, 1991. According to such a technique published, the dichroism beam splitter 3 is used while setting the incident angle to 10.degree. and an angle between the incident light beam 101 and the light beam 105 is set to 160.degree. and, in the above construction, by using a reflecting member, the optical parts in the optical system 301 using the semiconductor laser device 1 and the optical system 300 using the semiconductor laser device 6 are vertically or horizontally arranged, and the dichroism beam splitter 3 is arranged in substantially the same manner as the optical system which is used while setting the incident angle to 45.degree..
However, in the 2-laser optical head according to the former conventional technique (FIG. 2), the angle between the light beams 101 and 105 which enter the dichroism beam splitter 3 is set to 160.degree. instead of 90.degree.. Consequently, the arrangement of the optical parts of the optical system 301 using the semiconductor laser device 1 and the optical system 300 using the semiconductor laser device 6 becomes complicated and the whole optical system also increases in size. According to the latter conventional technique, the reflecting part to bend the optical path is necessary, the number of optical parts increases, and the costs of the optical head are inevitably high.